When functioning properly, the human heart maintains its own intrinsic rhythm based on physiologically-generated electrical impulses. It is capable of pumping adequate blood throughout the body's circulatory system. Each complete cycle of drawing blood into the heart and then expelling it is referred to as a cardiac cycle.
However, some people have irregular or other abnormal cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One mode of treating cardiac arrhythmias uses drug therapy. Drugs are often effective at restoring normal heart rhythms. However, drug therapy is not always effective for treating arrhythmias of certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of a cardiac rhythm management system. Such systems are often implanted in the patient and deliver therapy to the heart.
Cardiac rhythm management systems include, among other things, pacemakers, also referred to as pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart, such as via an intravascular leadwire or catheter (referred to as a “lead”) having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pace pulses (this is referred to as “capturing” the heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly. Such pacers coordinate atrial and ventricular contractions to improve pumping efficiency.
Cardiac rhythm management systems also include defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Such defibrillators also include cardioverters, which synchronize the delivery of such stimuli to portions of sensed intrinsic heart activity signals. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn't allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering a high energy electrical stimulus that is sometimes referred to as a defibrillation countershock, also referred to simply as a “shock.” The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, drug delivery devices, and any other implantable or external systems or devices for diagnosing or treating cardiac arrhythmias.
One problem faced by cardiac rhythm management systems is determining the rate at which pacing pulses are delivered to the heart. Some systems include one or more operational modes referred to as atrial tracking modes. In an atrial tracking mode, the system detects atrial heart contractions triggered by the heart's “physiological pacemaker,” referred to as the sinoatrial node. Based on the rate of the detected heart contractions, the system determines the rate of pacing pulses delivered to a ventricle. Such an atrial tracking mode is useful in a patient having a functional sinoatrial node, but where the patient has an atrioventricular node that fails to conduct physiological electrical impulses from the atrium to the ventricle. Some cardiac rhythm management systems further include one or more rateresponsive atrial tracking modes. In a rate-responsive atrial tracking mode, the system adjusts the rate of ventricular pacing pulses based not only on the rate of atrial heart contractions, but also based on a physiologic sensor-indication of the patient's metabolic need for increased blood circulation. One example of such a sensor is an accelerometer that detects the patient's activity for providing an indication of a need for a higher pacing rate. Another example is a ventilation/respiration sensor that detects the patient's breathing rate for providing an indication of a need for a higher pacing rate. In one implementation of a rateresponsive atrial tracking mode, the ventricular pacing rate is based on the detected atrial heart rate unless the physiologic sensor indicates a need for an even higher ventricular pacing rate.
In both the atrial tracking mode and the rate-responsive atrial tracking mode, the system typically includes a maximum atrial tracking rate (MATR). The MATR imposes a limit beyond which the ventricular rate does not track the detected atrial heart rate. The MATR limit prevents pacing the ventricle at a dangerously high rate during an atrial tachyarrhythmia. One example of a cardiac rhythm management device includes a MATR of 120 beats per minute (bpm) as a factory default value. The physician can adjust the MATR at the time that the cardiac rhythm management device is implanted in the patient. The present inventors have recognized that without additional clinical data informing the physician of an appropriate MATR value for a particular patient, the physician typically must settle for programming the MATR to an overly conservative value that may be inappropriately low for the particular patient, thereby limiting the patient's ability to exercise or engage in activity at higher heart rates. Moreover, cardiac rhythm management devices typically include a multitude of programmable parameters that can be adjusted by the physician at the time of implant, increasing the complexity and time required for the implant procedure. Accordingly, the present inventors have recognized that there is a need for a cardiac rhythm management system that automatically determines an appropriate MATR value.